In the machines for the injection moulding of plastic material, the injection units are provided for receiving the plastic material, in a molten or fluid state, from a feeding unit and for injecting it at high pressure into the hollow of a mould.
These injection units can be of various type and, for instance, they can be associated with a closure group or assembly, in turn comprising a closing rod or stem or pin which is adapted to selectively open or close, by the respective tip, an injection hole, often called also “gate”, in order to control the duration of the injection stage and hence the quantity of the fluid plastic material injected into the mould.
More in detail, in the injection units of this type, the closing rod is driven to move alternately, along its axis, between an open position, in which the tip of the closing rod set free and opens the injection hole, whereby the molten plastic material can freely flow into the hollow of the mould, and a closed position, in which such tip closes the injection hole so as to interrupt the flow of the molten plastic material from the injection unit toward the hollow of the mould.
This axial alternate movement of the closing rod is for instance driven and controlled by a hydraulic or pneumatic piston, which is associated with an end of the closing rod and is subject to the pushing action, along the two directions, of a fluid in pressure, as compressed air, in turn controlled, in the flow towards and from the pneumatic piston, by a valve system of known type.
The open position of the closing rod, in correspondence of which the injection hole is, as said, open and adapted to allow the free flow of the molten plastic material towards the mould, usually does not need to be accurately regulated.
By contrast, it is instead very important, in order to optimize and to obtain the maximum outcome of the operation of injection, to control and adjust with extreme care and precision the closing rod in the respective closed position, in which, as said, the tip of the closing rod closes the injection hole and thereby interrupts the flow of the molten plastic material towards the hollow of the mould.
At present there are in use a lot of different solutions, systems and devices, to adjust the closing rod in the respective closed position.
For instance, a known solution adopts a threaded nut, that is mounted on the pneumatic piston which drives the axial motion of the closing rod, with this being associated with a click mechanism comprising a ratchet or pawl, in turn adapted to be triggered elastically in a plurality of seats, whereby, by rotating this threaded nut, between a click and the other of the ratchet in these seats, it is possible to vary and to adjust the axial position of the closing rod with respect to the body of the injection unit.
This solution has, however, the drawback that it is constructively very complex because of the presence of the click mechanism, and moreover that it is cumbersome, since the threaded nut constitutes an additional piece which protrudes from the body of the piston that drives the closing rod.
Also it is known to adjust the closing rod in the respective closed position by using one or more elements of a determined thickness, i.e. by interposing them between the piston, movable axially, which drives the closing rod, and the area, of the fixed body of the injection unit, against which the same piston is pushed and enters into contact in correspondence of the closed position of closing rod.
This other solution is clearly uncomfortable and of complex actuation.